How difficult would it be to sue a contractor for gross negligence in delaying a project?

I am purchasing a building in which I will be opening a business. I had to hire an environmental engineer to sample the soil at the property as a stipulation of the mortgage.
On two separate occasions the engineer completed the contracted sampling, received the results from the lab, formulated the letter to accompany the results to be submitted to the DNR, and received approval of the letter from myself along with a request to submit the entire package to the DNR. After all of this was completed, it took the engineer 3 WEEKS to submit the information to the DNR (via email) on the first occasion, and now more recently 4 DAYS to submit the second round of sample results.
Other relevant information:
-invoices were all paid by me immediately upon receipt, so this delay couldn't be considered retribution for late payment.
-I can easily prove damages related to 25 days delay in opening the business.
-I was specifically asked by this engineer not to contact the DNR personally for any reason, otherwise I would have submitted the results myself.

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